JPS5927198B2 - Static variable speed reversible AC motor drive device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a variable speed AC motor 1 drive device configured using an inverter supplied with controlled current or voltage, and more specifically, a variable speed AC motor 1 drive device configured using an inverter supplied with controlled current or voltage, and more specifically, In order to do this, the present invention relates to an inverter frequency FbI leg system with a reversible variable frequency oscillator capable of unique multi-pulse operation.

Current inverter drives that utilize load angle control are one of several different types of static, adjustable frequency AC motor drives.

In such devices, when operating at low speeds, the desired frequency is a small DC component of the inverter frequency command signal. This command signal also has a sixth harmonic ripple with a large amplitude. This signal cannot be filtered out to maintain the required dynamic performance of the device. Both the desired frequency and the direction of rotation of the motor, whether forward or reverse, must be determined from this signal, and operation and reversal must be smooth over a wide torque range. When using angular control, unless special efforts are made, the drive device may enter into a pattern of repeated reversals, which makes the performance of the device completely unsatisfactory and risks damaging the mechanical load. Because of this, there is a risk that low-speed operation and reversing will become extremely unstable. Load angle control of a current controlled type inverter drive device and a pulse width modulated voltage type inverter drive device is described in Japanese Patent Application No. 52-118,690. In a developing transportation application, this problem was solved by a diode clipper that limited the dynamic range of the angular error signal before input to the proportional and integral controller. Although the drive device of this invention was developed for a different application, it does not include integration of the angular error signal and uses a simple diode.
It turned out that clamps were not the solution. It has been described in the past to operate current controlled and square wave voltage type inverters in a pulse width modulated manner and to mathematically optimize the timing of the pulses to eliminate various harmonics.

However, the common means of timing these pulses is to use counting logic circuits to generate the appropriate pattern in signal levels. By multi-pulse operation,
Although deleterious tooth profile torque at low speeds can be reduced or eliminated, previous attempts to combine multi-pulse operation with load angle adjustment have been unsuccessful. A reversible variable speed AC motor drive constructed using either a current controlled inverter or a square wave voltage controlled inverter has an improved variable frequency oscillator in the inverter frequency control circuit and is capable of operating over a wide torque range. Allows for smooth operation and reversal. The inverter frequency command signal supplied to the oscillator has a substantial ripple component and a DC component whose magnitude and polarity represent the rotational speed and direction of the motor. The oscillator includes an integrator and operates to generate timing pulses at a frequency that is related only to the level of the DC component in the input signal and, depending on the polarity, to generate a forward or reverse signal. A ring counter or equivalent, in forward and reverse phase sequence,
Generates the gate pulse for the main thyristor of the inverter. The oscillator and inverter frequency control circuit is particularly suitable for current source inverter 1 drives that utilize load angle control, but torque control also has the same advantages.

The problem of chatter that occurred with conventional methods, especially in high torque and low speed operations, as well as in the case of reversals at high or low torque and low speeds, is eliminated. Under these conditions, the frequency command signal is almost entirely ripple, or has a large ripple component. Furthermore, multi-pulse operation is inherent at low speeds, and the resulting inverter waveform has pulse width modulation characteristics, which reduces torque modulation and improves the operation of the inverter commutation circuit. A preferred type of reversible variable frequency oscillator has an integrator connected in parallel with a reset switch;
In combination with it, positive and negative comparators and logic gates are used.

These comparators and logic gates act to generate timing pulses as clock pulses for the shift register type ring counter. timing·
Pulses are sent to the directional flip-flop to generate left shift and right shift inputs in response to forward and reverse direction commands. An overcurrent signal interrupts the oscillator output. The oscillator may include a lowest frequency limit mechanism. The invention will be better understood from the following description of preferred embodiments with reference to the drawings.

Reversible variable frequency oscillators are used in variable speed AC motor drives where very smooth operation and reversal over a wide torque range is required.

The oscillator, as well as the inverter frequency control circuit of which it is a main component, is particularly suitable for current source inverter drives with load angle control, but can also be used in other regulators or other types of inverters. I can do it. In such devices, when operating at low speeds, the desired frequency is a small DC component in a large AC signal, and this signal cannot be filtered out to maintain the desired dynamic performance of the device. The oscillator described here uniquely solves several intractable problems and is inherently capable of multi-pulse mode of operation, as required for high performance drives. When the speed decreases, a more complex inverter waveform is generated to eliminate or minimize ripples at low frequencies that can cause COgging or excite mechanical resonances in the motor and load. Keep it to a minimum. A current controlled AC motor drive is shown in the simplified block diagram of FIG. 1 together with load angle control or torque control means.

In this device, AC or DC power is converted to variable amplitude DC power by a voltage converter 10, such as a phase-controlled AC/DC rectifier or a DC/DC chopper. Next, this power is converted into AC form by the current control type inverter 11, and the DC reactor 12 is connected to the DC link (DC link means that the DC output of the converter that converts AC to DC is input to the inverter. (refers to a DC connection circuit that couples to The multiphase, non-sinusoidal output current of the inverter has a frequency that is variable in magnitude with the DC link current, and is fed to an adjustable speed AC motor, such as the induction motor 13. Controlling the voltage VB, applied to the DC link by the voltage converter 10, adjusts the level of the DC link current (i.e. the DC current supplied from the converter through the DC link to the inverter) DO, Therefore, the stator current is adjusted, and when the operating frequency of the current controlled inverter 11 is controlled, the stator excitation frequency is adjusted. For actual operation of the entire motor and converter, a stabilizing feedback circuit must often be used to maintain normal motor flux and current levels. Common control methods achieve this stabilization through dynamic control of motor current and amplitude. The load angle control method utilizes the inverter frequency as the required device stabilization control, which is achieved by adjusting the phase angle between the motor current and the motor magnetic flux. This method synchronizes the inverter's firing pulses with the motor's back emf, but is similar to voltage-fed induction motors where additional current is essentially provided to match the motor's back emf with the inverter's back emf. It is something. For further details and specific examples of a static AC drive device using load angle feedback control, please refer to Japanese Patent Application No. 118690/1982. The drive device has an instantaneous flux, instantaneous torque and instantaneous load angle calculation device 14 which receives information from a motor current sensing device 15, an air gap flux coil 16 and provides a feedback signal to a control device 17. but,
It may or may not have a tachometer 18 to generate a rotor speed feedback signal. The air gap flux ψ in volts and the instantaneous electromagnetic torque Te are derived by the circuit described in the above-cited patent application. The instantaneous electromagnetic torque Te is defined as the product of the horizontal stator current and the horizontal air gap magnetic flux minus the product of the horizontal stator current and the horizontal air gap magnetic flux. To calculate the instantaneous load angle or torque angle, it is preferable not to calculate the actual phase angle θ between the motor current and the motor flux, but instead to calculate the approximate feedback signal (SinO′) E, . This is because this quantity increases monotonically over the entire operating range and is therefore a more suitable variable for angle adjustment schemes. This equivalent quantity is expressed by the following formula.
Other circuits for generating a sensed value of load angle are disclosed in the above-cited patent application and in Proceedings of the IEEE Industry Applications Society Annual Meeting, October 2-6, 1977, pages 609-615.
It is also described in a paper by the same inventor entitled ``Synchronous Control of a Static AC Induction Motor Drive Device'' published on page 1.

FIG. 2 shows in detail the load angle adjustment loop of an induction motor drive using a current controlled inverter, and other parts of the control device are omitted for clarity.

The front end voltage converter is a phase-controlled rectifier 1' powered by a 60 Hz three-phase voltage source, and the current-controlled inverter 11' is an automatic sequential commutation inverter as described in U.S. Pat. No. 3,980,941. It is a polyphase thyristor bridge inverter like . Current-controlled inverters are also known as current-source inverters or current-fed inverters; other types are also commonly known:
You can use it for this invention. Among them, there are a third harmonic auxiliary commutation type inverter using one commutation capacitor and an auxiliary impulse commutation type inverter using three commutation capacitors. The angular feedback signal 1n0e and the command or reference value SlnO, shown schematically as a voltage level manually set by the wiper of the potentiometer 20, are supplied to the analog adder 21, which calculates the angular error. Generate a signal.

After passing through the gain circuit 22, the angle error signal having a substantial ripple component and the positive or negative DC rotor speed feedback signal ω are sent to an analog adder 23, and the output of this adder is an inverter frequency command. A signal is generated. The action of the reversible variable frequency oscillator 24 in the inverter frequency control circuit 25 of the present invention is to generate a DC component of the input command signal, which is positive or negative depending on the rotational direction of the motor, at a frequency equal to six times the desired motor frequency. a timing pulse with
forward or reverse force direction signals. Timing pulses and direction signals are sent to ring counter 26 to generate thyristor gate pulses in either forward or reverse phase sequence. Gate drivers, which are normally included in this control circuit, are not shown in the figure. The main thyristors T1 to T6 of the inverter are numbered according to their firing order in the forward phase sequence, and commutation details have been omitted. Before proceeding with the explanation, it should be noted that torque feedback control, which acts to adjust the inverter frequency to minimize the error between the torque reference value Te* and the torque feedback signal Te, also controls the instantaneous torque and Since the feedback signals for the instantaneous load angle are of similar form, the same control circuit can be used. Angle reference signal 6 Angle feedback signal and angular error signal (these are the inputs and outputs of analog adder 21) under various load conditions 6 Particularly for low-speed operation with large torque and low-speed reversal with large torque or small torque In order to understand the problems that arise when

The reference angle signal (Figure 3a) is a DC signal with variable levels. The angular feedback signal typically consists of a DC component and a sixth harmonic ripple with a large amplitude. Light load (
At the time of FIG. 3b), the angular feedback signal is essentially all ripples, and at heavy loads (FIG. 3c), there is a large DC component and the signal is about 50% ripple. At the input of the analog adder, the angular feedback signal is negative and the reference signal is positive polarity. The heavily loaded angular error signal (FIG. 3b), generated by adding the DC reference signal 28 and the angular feedback signal 29, is shown by the thick line 30, and the DC component 31 is very small. When the motor speed is low, the tachometer feedback signal ω is also a relatively small DC signal.
The inverter frequency command signal, which is the sum of the angle error signal and the tachometer feedback signal, has a large-amplitude ripple component and a small DC component, 6 from which the desired motor frequency and desired motor rotation direction must be determined. The basic motor frequency is the mechanical speed at which the shaft rotates (Rp
It is an electrical parameter corresponding to When inverting at low speed when the torque is large or small, the tachometer feedback signal is small and it passes through zero, so that the inverter frequency command signal has a large ripple component and a small DC component. The large ripple component is not detrimental to the operation of the reversible variable frequency oscillator 24 and is actually used advantageously for multi-pulse operation when reversing the motor at low speeds. In an AC motor drive system using a current-controlled inverter using load angle feedback control, many conditions are applied to the variable frequency oscillator in addition to those already mentioned.

An angle control is required for stability at high and low speeds without load. The variable frequency oscillator must not degrade the dynamic performance of the overall device and must be designed with a minimum frequency limit of 6. It is desirable to include means for stopping the output of the oscillator when an overcurrent is detected. The variable frequency oscillator 6 must eliminate the chattering problems previously encountered with other systems, especially for low speed operation with high torque and low speed reversal with high or low torque. A simple economical solution is needed. When a gain change is introduced to the oscillator in any operating region, the other parts of the regulator respond in order to maintain the planned flux-torque profile of the motor in order to minimize torque ripple. Changes must be made. FIG. 4 is a block diagram of a preferred form of reversible variable frequency oscillator 24 and ring counter 26.

The new main components are an integrator 33 with a reset switch 34 connected in parallel, positive and negative comparators 35, 36 connected in parallel with each other at the output of the integrator, . A logic gate 37 is connected between both comparator and set switches 34, a directional flip-flop 38 whose set and set inputs are connected to the comparators, and a clock switch for logic gate 37 and ring counter 26. and a delay element 39 connected between pulse inputs. The ring counter is. It is an ordinary shift register with six stages connected as a reversible ring counter. Integrator 33 can be an integrated circuit operational amplifier with a feedback capacitor connected between the output terminal and the summing point, and reset switch 34 is a solid state switch that shunts the integrating capacitor. The inverter frequency command signal 41 becomes the input of the integrator 33, and the switch 3
4 is open, a ramp output 42 is generated that rises at a rate related only to the DC component of the input signal.

Depending on the polarity of this DC component, the input is a positive reference value +Vr.
ef or negative reference direct, Ef (this or comparator 35, 36
(set for). When integrating to a positive reference value, the output of comparison product 35 changes from low to high, and the signal is passed through OR logic gate 37 to
Pass to switch 34, closing the switch and resetting the feedback capacitor. At this time, as a result of the integrator output decreasing,
Since the comparator 35 no longer exceeds the positive reference value,
The state changes from high to low output, at which time a timing pulse 43 is generated. This timing pulse train has a frequency related to the amplitude of the DC component of the command signal, and the comparator 3
5 or the output of comparator 36, all timing pulses pass through OR circuit 37 and delay element 39 to the clock input of shift register 40. The timing pulse from the positive comparator 35 is sent to the set input of the direction flipflop 38, and the timing pulse from the negative comparator 36 is sent to the set input.

The Q output of this flipflop and its complement Q output are the forward and reverse signals 44, 45, one of which is high and the other low. These direction signals are sent to the left shift and right shift inputs of a shift register type ring counter 40. If the comparator that changes state is the same as described above, the flip-flop will remain in the same state. When another comparator changes its state, the flip-flop changes state, thus reversing the direction of rotation of the ring counter. In this way, the phase order of the motion machine is also reversed. The effect of delay element 39 is to delay the timing pulse slightly so that the 6 shift left and shift right commands enter the shift register before the clock pulse arrives. When the inverter frequency control circuit 25 and the reversible variable frequency oscillator 24 operate within the closed feedback loop system shown in FIG. Greatly increases utility. The inverter waveform then has pulse width modulation characteristics, which reduces torque modulation and improves the operation of the inverter commutation circuit. When a current control type inverter or a square wave voltage type inverter is used for a motor drive device, the fundamental frequency of 6
Torque pulsations of twice the frequency are generated. At low operating speeds, the torque pulsation frequency can be low enough to cause chopping or excite mechanical resonances in the motor and load. Due to the multi-pulse mode of operation,
Without changing the inverter's power circuit, the amplitude of the pulsations is reduced 6 and its lowest frequency is increased. If the inverter is a voltage source type, large harmonic currents may be generated by low-order voltage socket harmonics. 6 due to multi-pulse mode of operation
As the amplitude of these harmonic currents decreases, their lowest frequency increases. Another problem that can occur when operating any thyristor inverter at very low frequencies is that the commutation capacitors may discharge during long durations between commutations. For this reason, there may not be enough charge remaining to carry out the sixth-order commutation. Multi-pulse operation allows the inverter to operate at arbitrarily low fundamental frequencies while maintaining a maximum time between commutations of less than 1/3 of the period of the fundamental frequency. The invention implements a 6 multi-pulse mode of operation without the use of 6 special hardware in either the signal or power stages. When the operating frequency is very low, the tachometer feedback signal ωr (rod 2
(Fig.) is a DC level that is smaller or slightly smaller than the angle reference signal or the calculated load angle feedback signal. In these conditions, the angular error signal sent to analog summer 23 is the difference between a fixed reference and the sensed instantaneous Slnθ. Ignoring the small input of the tachometer, the integrator 33 integrates this error and shifts the ring counter 26 left or right when the integrator error reaches the level set by the comparator reference. let Each left or right shift of shift register 40 corresponds to firing the thyristors of the inverter in the rotational order of the phases corresponding to the direction of the shift register. By alternating left and right shifts, the six motor currents achieve the multi-pulse characteristics necessary to restore the desired waveform. The typical single phase inverter output current waveform shown in FIG. 5a includes current pulses of opposite polarity with a duration of 61,200. In the multi-pulse mode of operation shown in Figure 5b, there is a notch in the current waveform. That is, a large number of pulses are generated during 1200 periods.
These notches are at near-optimal locations to minimize torque errors. Timing pulses are generated at a rate typically greater than six times the commanded fundamental excitation frequency. In operation, the effect of variable frequency oscillator 24 is to shift forward or backward as required by the integrated error between the commanded angle and the instantaneous load angle.

That is. The oscillator acts as a Bang-Bang regulator to minimize the integrated error between the 6 reference angle and the commanded angle. The effect of this pulsing is to keep the desired angle as close to the reference angle as possible with the switching device. This circuit is. For any steady-state torque and speed, both the current 1d0 and the magnetic flux ψ are approximately constant, but considering that the torque Te can vary cyclically due to instantaneous fluctuations in the equivalent load angle θE, 6
This can be viewed as minimizing torque harmonics. In fact, this state is the normal state of current-controlled inverters. When the current and magnetic flux hardly change, the denominator DO(ψ-KI,O) of the angle equation is close to constant at a given load. At this time. The angular ripple is almost entirely a torque ripple, and the two oscillators are shifted back and forth to minimize torque errors. It can be shown that the pulse width modulation pattern that minimizes the integrated error is the theoretically ideal pattern that reduces harmonics. Testing has shown that this equipment meets the 1977 EEE Industry Applications Society International Semiconductor Power Converter Standard.
W. from the conference minutes, pages 295 to 304. It has been found that it is possible to generate the first three types of PWM waveforms described in the paper by Hajime Rhino et al., "Power converter for feeding an asynchronous traction motor of a single-phase AC vehicle." DC motor speed feedback signal ω, The action of is to bias integrator 33 to rotate in one direction.

That is, when the tachometer signal is large, the inverter responds normally and the phase rotation is continuous in one direction. When the tachometer signal decreases towards zero. Initially there is one extra pulse on each switching and then six additional pulses as the tachometer signal approaches zero. This automatically results in a change in the multi-pulse pattern, resulting in more complex waves as the velocity decreases. The present invention allows angle control and multi-pulse operation to be used interchangeably and allows the same signaling hardware to perform both functions. The reversible variable frequency oscillator and ring counter of FIG. 6 has been modified to include a minimum frequency limit and an overcurrent trip that shuts down the oscillator output.

As mentioned earlier, the input to the integrator 33 is the input to the comparator 35.
, 36 is integrated up to a positive or negative reference value. In this embodiment, the output of the integrator is coupled to a directional flip-flop 38 and also to a one-shot multivibrator 47 via an OR gate 37. If the comparator changing state is not the same comparator as before, the state of directional flip-flop 38 changes. The output of the comparator is sensed by a one-shot multivibrator 47, which acts as a delay element so that the flip-flop can be reset, if necessary, before the next pulse of the oscillator 6 occurs. Make it. This pulse of the oscillator is triggered by the one-shot multivibrator 4 which is triggered by the delayed output of the one-shot 47.
It is taken out from 8. One-shirotsudo multi pipe plater 4
The output of 8 passes through an OR gate 49 and an AND gate 50 and is fed back through an amplifier 51 to the set switch 34, which closes and shorts the integrating capacitor of the integrator 33, starting the next cycle. Set it to zero in case. The delay of the one-shot multivibrator 47 also acts to give the output of the comparator a finite pulse width. Timing pulse passed through AND circuit 50 triggerable one-shot multivibrator 5
2 is also input, and if the period from one shot 48 exceeds the period of one shot 52, one shot 52 triggers the fourth one shot manipulator 53.

This multipipulator 53 then acts as the low frequency limit of the oscillator. The timing pulses 43 generated by normal operation or by the lowest frequency limit circuits 52, 53 will not be activated unless the AND gate 50 is disabled by the overcurrent detection and trip circuit. ,
It is coupled to the clock input of a shift register type ring counter 40. A DC current sensing device 54 senses the DC link current or the inverter output current. A signal representative of the current level is provided to an overcurrent limit comparator 55. When the current signal exceeds the reference value, the output of the two comparators switches from low to high, generating an overcurrent signal. This overcurrent signal sets overcurrent trip flipflop 56 to an inhibited state and removes the enable input to anode circuit 50. At this time, timing pulse 43 is no longer coupled to the clock input of shift register 40 and the oscillator output stops. After an overcurrent or during start-up, the oscillator is manually re-initialized. A start/reset pulse is applied to the overcurrent flip-flop 56 reset input, changing the state of the output and regenerating the enable signal for AND gate 50. Direction signals 44, 45 generated at the output of direction flip-flop 38 as well as the complement output pass through a pair of OR circuits 57, 58 to right shift and left shift inputs of shift register 40. A resume/reset pulse is also provided through these logic gates to resume operation of the six shift registers. In summary, high-performance static AC motor drives of the current source or voltage source inverter type are characterized by smooth operation at low speeds as well as smooth reversal over a wide torque range. When the inverter frequency control circuit and variable frequency oscillator respond to a signal with large AC ripple, the DC component containing the desired motor speed and rotation direction information is small. Use that to your advantage. Six different control methods can be used including load angle adjustment and torque adjustment. The application of high-performance current-controlled drives using angle control is large industrial drives.

[Brief explanation of the drawing]

Figure 1 is a simplified block diagram of a static AC motor drive device using a current control type inverter and angle control.
The figure is a schematic diagram showing a part of an AC motor drive system using a current control type inverter as a block diagram.
The load angle adjustment loop is shown in detail, but other details have been omitted. Figures 3a to 3d are graphs showing the waveforms of the reference angle signal and the angle feedback signal under light load and heavy load conditions, and the waveform of the angle error signal with a small DC component and large AC ripple under heavy load conditions. Figure 4 is a block diagram of a reversible variable frequency oscillator and a shift register type ring counter.
Figures 5a and 5b are graphs showing the single-phase output current waveforms for normal and multiphase operation. Figure 6 is the reversible frequency modified to include overcurrent drag and lowest frequency limits. FIG. 2 is a detailed block diagram of an oscillator. Explanation of main symbols,
1V; inverter, T1 to T6: thyristor, 13:
AC motor, 24: Reversible variable frequency oscillator, 25: Inverter frequency control circuit, 26: Ring counter 633: Integrator.

Claims (1)

[Scope of Claims] 1. An inverter frequency control circuit that supplies alternating current power of suitable amplitude and adjustable frequency to the motor and generates gate pulses for the main thyristor of the inverter in response to an inverter frequency command signal. In a stationary variable speed reversible AC motor drive including an inverter, the inverter frequency control circuit changes according to the difference in magnitude between a reference value and a sensed motor parameter, and a reversible variable frequency oscillator receiving as input an inverter frequency command signal having a DC component whose magnitude and polarity represent the rotational speed and direction of the motor, the oscillator including an integrator; the inverter frequency control circuit is operative to generate timing pulses and direction signals at a frequency related only to the DC component of the input signal; In contrast, a static variable speed reversible AC motor drive having means for generating gate pulses in forward and reverse phase sequence. 2. In the stationary variable speed reversible AC motor drive device according to claim 1, the integrator generates a ramp output that increases at a rate related to the DC component of the input signal, and the oscillator: A static variable speed reversible AC motor drive including means for generating said timing pulse and direction signal when said integrator output repeatedly rises to a preset level and is reset. 3. A stationary variable speed reversible AC motor drive as claimed in claim 2, wherein the means for generating timing pulses and direction signals comprises positive and negative comparators coupled in parallel to the output of an integrator. a logic gate connected between the comparator, both comparators, and a switch that resets the integrator so that a timing pulse is generated at the output of the comparator. and a directional flip-flop operative to generate a forward or reverse direction signal. 4. In the static variable speed reversible AC motor drive according to claim 3, means for generating the gate pulse for the thyristor is coupled to the directional flip-flop and for generating the gate pulse for the thyristor through a delay element. It consists of a reversible ring counter that is also coupled to the output of a logic gate,
A static variable speed reversible alternating current motor drive device, wherein the timing pulse is used to shift the ring counter. 5. In the stationary variable speed reversible AC motor drive device according to claim 4, means for generating an overcurrent signal when the sensed inverter current exceeds a reference value, and a clock input of the ring counter. and another logic gate connected to inhibit passage of the timing pulse when the overcurrent signal occurs. 6. A stationary variable speed reversible AC motor drive as claimed in claim 4, wherein the clock pulse for the ring counter is actuated by the timing pulse and after a preset period of time. A static variable speed reversible alternating current motor drive having a minimum inverter frequency limit delay circuit. 7. In a static variable speed reversible AC motor drive according to claim 1, an angular error signal representative of the difference between the sensed instantaneous angle between the motor current and the motor flux and a reference value is provided. a load angle feedback control circuit that generates a load angle feedback control circuit, the angle error signal being a component of the inverter frequency command signal, and the inverter frequency control circuit being essentially multi-pulse at low motor speeds; A stationary variable speed reversible AC motor drive. 8. In the stationary variable speed reversible AC motor drive according to claim 7, the integrator has a reset switch, and the means for generating the timing pulse and direction signal is connected to the output of the integrator. positive and negative comparators coupled in parallel, connected between both comparators and the reset switch, such that the output of either comparator closes the switch, so that the comparator consisting of a logic gate for causing a timing pulse to be generated at the output and a corresponding directional flip-flop connected to both comparators to generate a forward or reverse signal;
Means for generating said gate pulses for a thyristor comprises a shift right and a left shift input coupled to said directional flip-flop and a clock pulse input connected via a delay element to an output of said logic gate. - A static variable speed reversible AC motor drive comprising a register type ring counter, the timing pulse being used to shift the ring counter. 9. In the static variable speed reversible AC motor drive device according to claim 8, in the load angle feedback control circuit,
The angular error signal is summed with a positive or negative DC motor speed feedback signal to generate an inverter frequency command signal, and at low motor speeds, the characteristic operation of the variable frequency oscillator is to generate frequencies greater than six times the commanded base excitation frequency. A static variable speed reversible alternating current motor drive that generates timing pulses and direction signals to generate current pulses in a pulse width modulated format to reduce torque pulsations. 10. A static variable speed reversible AC motor drive according to claim 1, further comprising a torque period control circuit for generating an error signal representative of the difference between the sensed sequential torque and a reference value; A static variable speed reversible AC motor drive device, wherein the error signal is one component of the inverter frequency command signal.